1. Field of the Invention
The present invention relates to a display device, more specifically, a display device in which gray scale display is made by both the voltage gray scale method and the time ratio gray scale.
2. Description of the Related Art
A technique that has recently accomplished rapid development is to manufacture a semiconductor device in which semiconductor thin films are formed on an inexpensive glass substrate, for example, a thin film transistor (TFT). This rapid development is caused by a growing demand for active matrix type display devices.
In an active matrix display device, a pixel TFT is placed in each of pixel regions as many as several hundred thousands to several millions arranged in matrix, and electric charge that flows into and out of a pixel electrode connected to each pixel TFT is controlled by the switching function of the pixel TFT.
As images are displayed with higher definition and higher resolution, demand for multi-gray scale display, desirably, in full color, has been established in recent years.
Accompanying the movement regarding display devices towards higher definition and higher resolution, the active matrix display device that has drawn attention most is a digital driven active matrix display device that can be driven at a high speed.
The digital driven active matrix display device needs a D/A converter circuit (DAC) for converting digital video data inputted from the external into analogue data (voltage gray scale). There are various kinds of D/A converter circuits.
The multi-gray scale display capability of the digital driver active matrix display device is dependent on the capacity of this D/A converter circuit, namely, how many bits of digital video data the D/A converter circuit can convert into analogue data. For instance, in general, a display device having a D/A converter circuit that processes 2 bit digital video data is capable of 22=4 gray scale display. If the circuit processes 8 bit data, the device is capable of 28=256 gray scale display, if n bit, 2n gray scale display.
However, enhancement of the capacity of the D/A converter circuit costs complicated circuit structure and enlarged layout area for the D/A converter circuit. According to a lately reported display device, a D/A converter circuit is formed on the same substrate where an active matrix circuit is formed, using a polysilicon TFT. In this case, the structure of the D/A converter circuit is complicated to lower the yield of the D/A converter circuit, resulting in yield decrease of the display device. In addition, increased layout area of the D/A converter circuit makes it difficult to downsize the display device.